refactor: rename package scope from @caldav to @calchat

Rename all workspace packages to reflect the actual project name:
- @caldav/client -> @calchat/client
- @caldav/server -> @calchat/server
- @caldav/shared -> @calchat/shared
- Root package: caldav-mono -> calchat-mono

Update all import statements across client and server to use the new
package names. Update default MongoDB database name and logging service
identifier accordingly.

apps/client/package.json

@@ -1,5 +1,5 @@
 {
-  "name": "@caldav/client",
+  "name": "@calchat/client",
   "main": "expo-router/entry",
   "version": "1.0.0",
   "scripts": {
@@ -12,7 +12,7 @@
     "build:apk": "eas build --platform android --profile preview --local"
   },
   "dependencies": {
-    "@caldav/shared": "*",
+    "@calchat/shared": "*",
     "@expo/vector-icons": "^15.0.3",
     "@react-navigation/bottom-tabs": "^7.4.0",
     "@react-navigation/elements": "^2.6.3",